A power chuck or mandrel such as described in my numerous earlier patents has a chuck or mandrel body centered on and rotatable about the axis of a headstock on which it is mounted. Normally jaws or similarly effective elements on the chuck body can be displaced radially by axial displacement of a central operating member. In a standard chuck this operating member may have a plurality of hooks with inclined faces that engage oppositely inclined faces of the jaws so that displacement of the hooks cams the jaws radially in or out depending on the axial displacement direction. In a power mandrel the operating member can be a piston which pressurizes the interior of an outwardly swellable sleeve that engages the inner surface of a tubular workpiece or of a bore in a workpiece or tool.
Such a chuck can be actuated by a device of the type described in my now abandoned U.S. patent application Ser. Nos. 580,460 and 580,461 both filed Feb. 15, 1984. This arrangement has a cylinder extending along an axis and a piston axially displaceable in the cylinder and forming therein two compartments. The cylinder has an axially backwardly extending stem also centered on the axis and formed with two respective passages each having one end opening into a respective one of the compartments and another end opening at a respective location on the stem, these locations being axially offset from each other. A connector rotatable about the axis on the stem is provided with respective fittings communicating with the locations which are normally constituted as radially outwardly open grooves into which the ends of the respective passages open. Bearings prevent the connector from moving axially on the stem. Fluid is fed under pressure alternately to the fittings to pressurize and depressurize the respective compartments and thereby urge the piston in respective axial directions in the cylinder. The cylinder is connected to a chuck body and the piston to a chuck-actuating member for fluid actuation of the member. A respective double check valve in each of the passages inhibits flow out of the respective compartment unless the other compartment is pressurized, so that neither compartment can drain unless the other compartment is pressurized.
The play between the outer surface of the stem and the inner surface of the connector is a few hundredths of millimeters and a tiny bit of leakage is permitted from the feed grooves along this interface to keep it and the support bearings lubricated. Thus liquid leaks along the joint to the two bearings, passes through them, and is collected in a sump or catchment formed in the bottom of the connection which itself does not rotate with the actuator.
In another known system small passages extend from the inlet passages in the connection to compartments adjacent the bearings, so the hydraulic fluid can feed thence directly to these bearings.
A main problem with either arrangement is that the path from whichever groove is pressurized to one bearing is much longer than the path from this pressurized groove to the other bearing. In addition an unpressurized groove will lie between the pressurized groove and its bearing at any time. Thus the flow will be in only one direction along the stem-connection interface. If the chuck or mandrel is frequently opened and closed there are no problems, but if it is held for long periods in one or the other position without change of actuation direction, there is a serious risk of the one end of the interface and the corresponding bearing drying out completely.
Furthermore at high rotation speeds the thin film filling the interface heats up considerably, principally because of the considerable shear it is subjected to between two closely spaced relatively moving surfaces. It therefore becomes ineffective as a coolant and lubricant. This problem can be partially solved by shortening the axial length of this interface to ensure considerable flow through it. When there is such substantial flow, however, at high speeds the fluid is ejected from the end of the unit where it is vaporized, creating a particular nuisance for the nearby operator and equipment.